1. Technical Field
This invention relates to a process for effectively removing harmful substances essentially consisting of inorganic and/or organic chlorides remaining in a waste gas exhausted from a chlorination furnace during the manufacture of titanium tetrachloride or zirconium tetrachloride.
2. Background Art
In general, titanium tetrachloride or zirconium tetrachloride is manufactured by heating raw materials such as a natural rutile, synthetic rutile, titanium slag, ilmenite, lucoxine, zircon sand, zirconium oxide and the like at a temperature of about 1,000.degree. C. in the presence of a suitable reducing agent such as a petroleum coke in a chlorination furnace so that the metal oxides may be reacted with chlorine. A waste gas exhausted from the chlorination furnace during such process contains CO and a small amount of various chlorides in addition to a large quantity of CO.sub.2. These chlorides consist of chlorides of various metals contained in the raw materials, and also chlorides of hydrogen, oxygen, carbon, sulfur and the like. In addition, when the reaction in the furnace is unsatisfactory, the waste gas sometimes contains unreacted free Cl.sub.2 as well. Accordingly, such harmful substances as the chlorides remaining in the waste gas must be removed from the waste gas prior to its exhaustion to the atmosphere.
One of the purification processes conventionally used to remove such waste chlorides from the waste gas comprises the step of scrubbing the gas with water and/or alkaline solution. Such process consists in contacting the chlorides with water to hydrolyze most of the chlorides to HCl, hydroxide and acid chlorides, and has been widely practiced in industry.
However, this process is not satisfactory for reducing the concentration of chlorides remaining in the waste gas to a level of, for example, no more than 5 ppm in HCl equivalent according to the current regulation in Kanagawa prefecture, Japan. Furthermore, the scrubbing of waste gas with an alkaline solution produces carbonate which is hardly soluble in water and causes the cleaning apparatus to get clogged with the carbonate because the waste gas essentially consists of CO.sub.2 and a further process is required to remove the carbonate therefrom for disposal.
In the manufacture of titanium tetrachloride, there is proposed a waste gas cleaning process which comprises the steps of adsorbing relatively larger quantities of titanium tetrachloride contained in the waste gas in an activated carbon and then scrubbing the waste gas with water and alkaline solution. However, this process is adapted mostly for the purpose of recovering titanium tetrachloride and is unsatisfactory in that the activated carbon rapidly becomes exhaused in its adsorption capacity owing to the existence of relatively larger quantities of titanium tetrachloride to be adsorbed, and it must be frequently subjected to a troublesome regeneration process. Furthermore, the concentration of chlorides remaining in the waste gas can only be reduced to a level equivalent to that in the conventional cleaning process using water and alkaline solution.
Accordingly, the present invention is intended to eliminate the above-mentioned disadvantages of the prior art. It is an object of the present invention to provide a process for the treatment of a waste gas from a chlorination furnace which is capable of remarkably reducing the content of chlorides remaining in the waste gas.
The inventors have conducted extensive research and experimentation in order to accomplish the above-mentioned object and have completed this invention based on such findings that adsorption capacity of activated carbon can be significantly increased so as to make it possible to reduce content of chlorides remaining in a waste gas to a level of no more than 5 ppm in HCl equivalent which is difficult to attain in a process of the prior art if moisture content of the waste gas after scrubbing with water is reduced to a level of no more than a predetermined level in view of the fact that the moisture content of the waste gas adversely affects the adsorption capacity of activated carbon, although the activated carbon can be used without decreasing the adsorption capacity for a relatively long period of time if the chlorides are removed from the waste gas as much as possible by scrubbing the same with water followed by adsorbing the chlorides remaining in a small amount in the waste gas in the activated carbon.